The removal from fluids of urea-like waste substances such as those which must be eliminated from the body in the urine plays a particularly important role in the extra-renal cleansing of the blood in artificial kidneys; however, this problem also arises in other technical fields, such as the removal of urea, ammonia and the like from the water in swimming pools and aquariums.
In medical dialysis technology, the tendency is toward appliances which are as compact as possible and thus portable. The volume of the dialysis medium is small, and the dialysis medium or hemofiltrate must be continuously regenerated in order to remove the waste product from it. This regeneration is accomplished inside a cartridge which contains compounds capable of decomposing or absorbing the waste products. Because urea and other substances which are normally eliminated from the body in the urine have a low capacity for reacting chemically under physiological conditions, the problem of removing these compounds from the dialysis medium and the hemofiltrate has not yet been solved satisfactorily.
There are appliances on the market in which urea is converted into ammonia by means of urease, and other substances normally eliminated from the body in the urine are removed by absorption means or by ion exchangers. Activated charcoal serves as absorption means, and zirconium phosphate and zirconium oxide, for example, are used as ion exchangers. The disadvantage in this case is that urease is sensitive to heavy metals, so that cleansing to remove all enzymatic toxins must be carefully performed. This requires taking expensive additional steps and increases the volume of the cleansing cartridge which is needed.
It is furthermore known to use complexing compounds bonded to a polymer, in order to attain this object. According to German Disclosure Document DE-OS No. 27 34 741, xanthhydrol or a xanthhydrol derivative is bonded to a natural or synthetic polymer, such as polystyrene. The paper by Owen et al entitled "Hemoperfusion, Dialysate, Diafiltrate Purification", from the symposium at Tutzing (Federal Republic of Germany), Sept. 11-13, 1978, discloses the bonding of phloroglucin and dihydroxybenzoic acid to ion exchangers, such as Sephadex, and the utilization of these polymer-bonded complexers for the purpose of urea absorption.
The attempt has also been made to use macromolecular compounds, such as oxygenated starch and oxycellulose, directly in order to remove urea from the dialysis medium via chemical reactions. However, this has had unsatisfactory results (see the paper, referred to above, from the Tutzing symposium).
The polymer-bonded complexes disclosed have various disadvantages. For instance, their absorption capacity is comparatively small, because the polymer component does not contribute anything to the absorption results. For the polystyrene compound known from the German Disclosure Document cited above, the calculation is that 1 g of the compound is theoretically capable of bonding 90 mg of urea. In the case of the Sephadex compound known from the Owen paper, the data given (which are, in part, contradictory) indicate a theoretical capacity between 16.5 and 33 mg/g. It is furthermore disadvantageous that the known polymers are chain-linked, one-dimensional high polymers in which the danger is extremely likely that products of separation will be released which cannot be metabolized in the human body. One disadvantage of the known products, which should not be underestimated, is that the cost of manufacturing them is comparatively high; there is also the fact that they are just as incapable of bonding phenols and phenolic and aromatic amino acids, which in patients suffering from uremia must also be eliminated from the body, and middle-sized molecules (300 to 20,000 Daltons) as they are of bonding Mg.sup.2+, K.sup.+, PO.sub.4.sup.3-, SO.sub.4.sup.2-, creatinine and uric acid.